With this example the constrained attribute is the same like the one
used in the LDAP URL. Now how are entries handled which are search
results of the LDAP URL? In this example how to deal with new values of
attribute 'ou' in case of adding a new entry with object class
'organizationalUnit'?

I would expect the source data for the constraint to reside in a separate DIT,
outside the constraint overlay's control.